Liquid delivery systems are used in aircraft primarily for fire fighting. Some types of delivery systems incorporate spray nozzles and these may also be used for crop spraying and the like. But for most fire fighting applications it is necessary to deliver a large quantity of liquid in a short time and this requires opening doors. It is necessary to ensure that liquid from a tank in an aircraft, either fixed wing or other, drops the liquid in a predetermined ground pattern to ensure the best possible coverage to extinguish the fire. Different drop patterns are desirable for different types of fires, for example, in forest fires a high concentration of liquid per area is required, whereas in brush fires and the like a more spread out drop pattern is desirable.
In fixed wing aircraft liquid tanks can contain up to 2500 gallons or more and because these aircraft are moving at a reasonably high air speed, it is necessary to ensure that the drop or discharge occurs over a short time interval to ensure that the liquid is concentrated in the drop pattern. This requires large opening doors otherwise the liquid is spread over a large area and thus does not have the same extinguishing abilities. In one embodiment there is a requirement for approximately one-half gallon per hundred square feet of liquid for grass fires whereas this increases to about four gallons per hundred square feet for forest fires. Most systems do not provide a variable flow rate. In one embodiment this is done by having a large number of doors and varying the door openings.
The door mechanisms for tanks used for aerial liquid delivery generally open outward. While this may provide a fast discharge of the liquid from within the tank it does not lend itself to controlling liquid flow. The doors open and all the liquid from the tank is immediately discharged. It is not easy to close the doors while there is still liquid in the tank as the doors have to close against the liquid discharging from the tank which requires a high force. Furthermore, with outward opening doors it is difficult to provide a suitable seal and in some cases the liquid in the tank tends to leak from the doors even while an aircraft is in flight.
Another problem with outward opening doors, and this particularly occurs with helicopters, is that the doors when open extend down below the landing wheels or skids and thus in certain situations prevent the aircraft landing with the doors open. This can present a problem if the doors become stuck in the open position. Alternatively, one must raise the tank up to ensure that this interference does not prevent the aircraft landing with the doors in the open position, and this may restrict the size of tank that can be carried by the aircraft.